JCL: Unit and Volume Affinity

When two or more volumes are assigned the same device, the volumes are said to

have unit affinity within the same job step allocation. Unit affinity implies deferred

mounting for all except one of the volumes.

A related concept is volume affinity. When two or more data sets share one or more

volumes, the data sets have volume affinity.

For concatenated data sets, code the following to assign the data sets to the same

device:

//DD1 DD DSNAME=dataset1,...

// DD DSNAME=dataset2,UNIT=AFF=DD1,...

// DD DSNAME=dataset3,UNIT=AFF=DD1,...

When you use explicit unit affinity, it is recommended that you use UNIT=AFF to

reference the previous DD in the unit affinity chain, rather than the primary DD. That

is, code:

//DD1 DD DSNAME=dataset1,...

//DD2 DD DSNAME=dataset2,UNIT=AFF=DD1,...

//DD3 DD DSNAME=dataset3,UNIT=AFF=DD2,...

//DD4 DD DSNAME=dataset3,UNIT=AFF=DD3,...

rather than:

//DD1 DD DSNAME=dataset1,...

//DD2 DD DSNAME=dataset2,UNIT=AFF=DD1,...

//DD3 DD DSNAME=dataset3,UNIT=AFF=DD1,...

//DD4 DD DSNAME=dataset3,UNIT=AFF=DD1,...

Always referencing the previous DD means that, if any condition causes the system

to ignore unit affinity for one of the DDs in the chain, any subsequent DDs in the

chain will still be allocated to a single unit, rather than to different units.